A large proportion of all energy consumed and generated is electrical energy. Electrical energy can be converted into a range of current-voltage combinations; on the other hand, in terms of chronological sequence, electrical energy can be converted from DC voltage to AC or three-phase voltage (with variable or fixed frequency) and vice versa. This conversion is now carried out mainly using power electronic systems, called converters.
The ongoing development of semiconductor technology, which was first introduced into power electronics about 30 years ago, enables and supports the production of such converters for ever expanding power and voltage ranges. Today, for example, switching of electrical currents and voltages in the medium voltage range is assured mainly with IGBTs and IGCTs. As semiconductors and the production means therefor undergo constant development, the reliability and power density of these components have also increased significantly.
Advances in the development of semiconductors also have beneficial effects for the development of novel converter topologies. Besides the high-voltage direct current (HVDC) transmission systems in the high-voltage range, which have until now incorporated thyristor circuits, attention has shifted to multilevel converters with significantly improved properties in terms of energy transmission, voltage or frequency conversion, and power factor correction.
Particularly in applications in the energy supply field, the switching voltages for converters are considerably higher than the inverse voltages of available power semiconductors. Consequently, application areas of this kind make use of circuit topologies that enable uniform distribution of the high voltage among multiple switching elements. One obvious method is direct switching in series of power semiconductors in converter branches. For this reason, each converter phase often consists of a series circuit of semiconductor switches, wherein the converter needs an intermediate circuit storage element in the form of a capacitor connected directly to the high voltage of the intermediate circuit.
However, these converter configurations become more and more complex as voltages increase, because the voltages must be distributed uniformly to the series-connected semiconductors and corresponding protection measures are needed to prevent overvoltages in the individual semiconductors.
In the event of a malfunction, extremely high discharge currents can flow through the capacitor in the intermediate circuit, and these in turn can lead to irreparable damage as a result of powerful mechanical forces and/or flashover damage.
A further disadvantage consists in that variable voltages can only be generated with drives of such kind with the aid of corresponding PWM conversion duty cycles, so that large voltage differences result in unfavourable duty cycles.
Many disadvantages of conventional converters can be solved by the modular multilevel converter such as is described in greater detail by R. Marquardt in DE 102 17 889. This system is capable of converting a practically limitless range of voltage characteristics from the terminal pairs on one side into a similarly wide range of voltage characteristics between the terminal pairs on the other side without having to differentiate between an input and an output according to the principle thereof. In the modular multilevel converter described by R. Marquardt, each phase of the converter is constructed from a large number of identical single modules connected in series. FIG. 1 shows a series circuit consisting of three identical modules 101, 102, 103 that make up a bridge branch 104 of a modular multilevel converter. Each single module functions as a two-terminal network and includes an energy storage element as well as a plurality of switching elements that can selectively hold or release current for both voltage directions, thereby achieving all four quadrants of the current voltage graph. FIG. 2 shows an embodiment of a single two-terminal network of a modular multilevel converter. A diode 205 to 208 is connected in parallel to each of transistors 201 to 204. The transistors can electrically connect output terminals 210 and 211 to capacitor 209. These single modules may particularly be switched to the following four states via their switches:                setting a positive terminal voltage for any current direction;        setting a negative terminal voltage with any current direction;        bypass state (that is to say no energy is taken up or released by the individual module), free current flow in any direction;        forced energy take-up by setting the voltage level.        
Consequently, such an individual module is already capable—given appropriate control, for example with clocked switching of the active elements (possibly analogously with pulse width modulation)—to control its own energy uptake and release as required, and to approximately simulate a virtual load with certain properties to a source. These modules may now be connected together for full four quadrant operation for n sources (for example two incoming voltage systems) and m outputs (for example a three-phase low-voltage system) as desired for a given application.
A combination of two serial circuits, each consisting of z modules is referred to as a phase module, wherein each of the two serial circuits forms a “bridge branch”. FIG. 3 shows a phase module 303 consisting of two bridge branches 301 and 302 in which the bridge branches are constructed from individual modules. The number z of modules in each bridge branch defines the voltage and harmonic properties of the converter. The phase modules in turn form the basic modules of a single- or multiphase power converter. Thus for example, as shown in FIG. 4, a system may be used to convert a single-phase AC or DC voltage to another single-phase AC or DC voltage via two connected phase modules 401 and 402. At the same time, the design of such a system is perfectly symmetrical in terms of inputs and outputs, thus enabling full four quadrant operation with respect to each connected pair. Moreover, the behaviour of the converter may be individually adapted with regard to inductive or capacitive behaviour at both the input and the output thereof. This means that energy can flow in both directions and can be altered dynamically.
Further, as shown in FIG. 5, a system for converting a three-phase AC voltage to a single-phase AC voltage or a DC voltage for example may be created with three connected phase modules 501, 502 and 503. The combined connections of the phase modules may also be thought of as a (DC voltage) busbar, so that n+m phase modules may be interconnected to create a network coupling for coupling an n-phase network with an m-phase network. FIG. 6 illustrates an exemplary interconnection of 5 phase modules 601 to 605 to form a coupling between a three-phase network and a two-phase network.
If certain areas of the current-voltage domain are sufficient, that is to say if full four quadrant operation is not required, connection arrangement may be simplified correspondingly.
Unlike a simple PWM conversion, which is only able to switch two voltage levels (0 and the full input voltage), from a source and accordingly can only process them in terms of timing and smoothing, the system of the modular multilevel converter is able to generate various stable voltage states as a function of the number z of modules, in the equivalent case 2z+1. Consequently, very fast voltage characteristics can be generated extremely precisely and with a very small harmonic component through corresponding high-frequency timing. Yet a pure step approximation at the same time is also possible.
A further advantage of the modular multilevel converter consists in that the energy storage units of the converter are located in the individual modules and no longer have to be designed as a single, large storage capacitor. This means that power converters with this converter topology can be constructed without a single large DC voltage intermediate circuit, through which extremely large short circuit currents may flow in the event of a fault. In conjunction with corresponding diodes over the switches, the storage capacitors of the individual modules also serve to damp possible voltage peaks extremely effectively, to protect the semiconductors for example. Consequently, unlike other converter topologies the inputs and outputs do not have be wired with additional capacitors, the insulating capabilities of which must be capable of sustaining the total maximum voltage.
Moreover, the four-quadrant operation of this converter type also enables applications such as power factor correction.
The construction of the converter from a plurality of identical individual modules also offers redundancy, so the functional capability of the converter can still be assured without additional switching devices if one or more of said two-terminal networks fails.
Compared with other converter topologies, the modular multilevel converter has the further advantage that the components of the respective modules do not have to be designed to sustain the full maximum voltage level of the input and output, they only have to isolate the module voltages. For many application fields of this converter type, this feature is financially very significant and it means that for the first time ever semiconductors can be used for these purposes.
In simple terms, the system relies on the controllable inclusion of modules in series. Modules that are not needed for generating a given voltage level are switched to the bypass state, so the energy storage unit keeps the charge it has at the time. But this represents a large unused potential. This occurs for example when the converter is only required to generate a relatively low voltage, so only certain of the modules are switched to the active state.
Particularly in application fields in which a relatively large voltage ratio is to be generated between the input and the output, or where low voltages occasionally have to be absorbed or released with high currents, this leads to unfavourable switching states and relatively high power losses in the components concerned.
Instead, the entire system must be designed to be able to sustain maximum voltages, which are seldom used; at the same time, the system must also be designed to handle the maximum current, although the current and voltage do not reach their respective maximum values at the same time in all application cases. The energy storage units used must also be designed for the maximum current demand, so they too must be overdimensioned.
In medical applications in which a converter serves as the source for a stimulation coil for inductive nerve stimulation, the stimulation coil also represents a highly inductive load. This means that the highest currents occur with very low voltages. Consequently, the individual modules and their energy stores are not used to the best effect in these applications either.
The matrix addressing approach for modules offers one possible way to mitigate these problems and to allow selective parallel connection of individual modules, while at the same time enabling the energy storage units of individual units to be switched between parallel and serial connection. However, this extremely desirable, maximum flexibility is bought at the price of a large number of required semiconductor switches, most of which must also be able to deal largely with the maximum total voltage level.
However, this high price does not appear to be practical except for research applications.